Manufacturing method of semiconductor device

ABSTRACT

A transistor with superior electric characteristics is manufactured. An oxide insulating film is formed over a substrate, an oxide semiconductor film is formed over the oxide insulating film, heat treatment is then conducted at a temperature at which hydrogen contained in the oxide semiconductor film is desorbed and part of oxygen contained in the oxide insulating film is desorbed, then the heated oxide semiconductor film is etched into a predetermined shape to form an island-shaped oxide semiconductor film, a pair of electrodes is formed over the island-shaped oxide semiconductor film, a gate insulating film is formed over the pair of electrodes and the island-shaped oxide semiconductor film, and a gate electrode is formed over the gate insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a semiconductor devicewhich includes a circuit including at least one semiconductor elementsuch as a transistor, and a manufacturing method thereof. For example,embodiments of the present invention relate to an electronic devicewhich includes, as a component, any of a power device mounted in a powercircuit, a semiconductor integrated circuit including a memory, athyristor, a converter, an image sensor, or the like, an electro-opticaldevice typified by a liquid crystal display device, and a light-emittingdisplay device including a light-emitting element.

2. Description of the Related Art

Transistors formed over a glass substrate or the like are manufacturedusing amorphous silicon, polycrystalline silicon, or the like, astypically seen in liquid crystal display devices. Although transistorsincluding amorphous silicon have low field effect mobility, they can beformed over a larger glass substrate. On the other hand, althoughtransistors including polycrystalline silicon have high field effectmobility, they are not suitable for being formed over a larger glasssubstrate.

In contrast to transistors including silicon, attention has been drawnto a technique by which a transistor is manufactured using an oxidesemiconductor, and such a transistor is applied to an electronic deviceor an optical device. For example, Patent Document 1 and Patent Document2 disclose a technique in which a transistor is manufactured using zincoxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and thetransistor is used as a switching element or the like of a pixel of adisplay device.

Meanwhile, it has been pointed out that hydrogen is a carrier sourceparticularly in an oxide semiconductor. Therefore, some measures need tobe taken to prevent hydrogen from entering the oxide semiconductor atthe time of depositing the oxide semiconductor. Further, variation of athreshold voltage is suppressed by reducing the amount of hydrogencontained in not only the oxide semiconductor but also a gate insulatingfilm in contact with the oxide semiconductor (see Patent Document 3).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055-   [Patent Document 3] Japanese Published Patent Application No.    2009-224479

SUMMARY OF THE INVENTION

However, conventional transistors including oxide semiconductors havelow on/off ratio, and do not provide sufficient performance as switchingelements in pixels of display devices. Further, the conventionaltransistors including oxide semiconductors are problematic because theirthreshold voltages are negative and they have normally-oncharacteristics.

It is an object of one embodiment of the present invention to providetransistors with superior electric characteristics.

One embodiment of the present invention is as follows: an oxideinsulating film is formed over a substrate, an oxide semiconductor filmis formed over the oxide insulating film, heat treatment is thenconducted at a temperature at which hydrogen contained in the oxidesemiconductor film is desorbed and part of oxygen contained in the oxideinsulating film is desorbed, then the heated oxide semiconductor film isetched into a predetermined shape to form an island-shaped oxidesemiconductor film, a pair of electrodes is formed over theisland-shaped oxide semiconductor film, a gate insulating film is formedover the pair of electrodes and the island-shaped oxide semiconductorfilm, and a gate electrode is formed over the gate insulating film.

One embodiment of the present invention is as follows: an oxideinsulating film is formed over a substrate, a pair of electrodes isformed over the oxide insulating film, an oxide semiconductor film isformed over the pair of electrodes and the oxide insulating film, heattreatment is conducted at a temperature at which hydrogen contained inthe oxide semiconductor film is removed and part of oxygen contained inthe oxide insulating film is desorbed, then the heated oxidesemiconductor film is etched into a predetermined shape to form anisland-shaped oxide semiconductor film, a gate insulating film is formedover the pair of electrodes and the island-shaped oxide semiconductorfilm, and a gate electrode is formed over the gate insulating film.

As the oxide insulating film formed over the substrate, an oxideinsulating film from which part of oxygen is desorbed by heat treatmentis used. The oxide insulating film from which part of oxygen is desorbedby heat treatment is preferably an oxide insulating film which containsoxygen with a higher proportion than a proportion of oxygen in thestoichiometric composition. Typical examples of the oxide insulatingfilm from which part of oxygen is desorbed by heat treatment includefilms of silicon oxide, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum oxynitride, gallium oxide, hafnium oxide,yttrium oxide, and the like.

In TDS (thermal desorption spectroscopy) analysis, the amount of oxygendesorbed from the oxide insulating film from which part of oxygen isdesorbed by heat treatment is 1.0×10¹⁸ atoms/cm³ or more, preferably1.0×10²⁰ atoms/cm³ or more, more preferably 3.0×10²⁰ atoms/cm³ or moreat the oxygen atomic conversion.

The temperature at which hydrogen is desorbed from the oxidesemiconductor film and part of oxygen contained in the oxide insulatingfilm is diffused into the oxide semiconductor film is 150° C. or higherand lower than the strain point of a substrate used, preferably 250° C.to 450° C.

The oxide semiconductor film is formed over the oxide insulating film,and the heat treatment is conducted at a temperature at which hydrogencontained in the oxide semiconductor film is desorbed and part of oxygencontained in the oxide insulating film is desorbed, whereby part ofoxygen contained in the oxide insulating film can be diffused into theoxide semiconductor film and hydrogen contained in the oxidesemiconductor film can be desorbed. Note that the oxygen diffused intothe oxide semiconductor film compensates for oxygen vacancies in theoxide semiconductor film, and thus the oxygen vacancies in the oxidesemiconductor film are reduced. In addition, the oxygen semiconductorfilm serves as a blocking film against oxygen desorption out of theoxide insulating film, and thus oxygen is not desorbed so excessivelyfrom the oxide insulating film and oxygen remains in the oxideinsulating film. In this manner, the concentration of hydrogen and thenumber of oxygen vacancies serving as carrier sources are reduced anddefects at the interface between the oxide semiconductor film and theoxide insulating film can be reduced.

By a bond of an element contained in an oxide semiconductor andhydrogen, part of hydrogen serves as a donor to generate electrons ascarriers. In addition, oxygen vacancies in the oxide semiconductor alsoserve as donors to generate electrons as carriers. For that reason, byreduction of the concentration of hydrogen and the number of oxygenvacancies in the oxide semiconductor film, a shift to a negative side ofa threshold voltage can be suppressed.

Further, defects at the interface between the oxide semiconductor filmand the oxide insulating film can be reduced by diffusion of part ofoxygen into the oxide semiconductor film and at the same time the otherpart of oxygen remains in the oxide insulating film, so that a shift toa negative side of a threshold voltage can be suppressed.

Note that in this specification, an n-channel transistor whose thresholdvoltage is positive is defined as a normally-off transistor, while ap-channel transistor whose threshold voltage is negative is defined as anormally-off transistor. Further, an n-channel transistor whosethreshold voltage is negative is defined as a normally-on transistor,while a p-channel transistor whose threshold voltage is positive isdefined as a normally-on transistor.

A transistor is formed as follows: an oxide semiconductor film is formedover an oxide insulating film, heat treatment is conducted at atemperature at which hydrogen contained in the oxide semiconductor filmis desorbed and part of oxygen contained in the oxide insulating film isdesorbed, and then the oxide semiconductor film is etched into apredetermined shape. The transistor formed in this manner can havesuperior electric characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are cross-sectional views illustrating a process formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIG. 2 is a top view illustrating a process for manufacturing asemiconductor device according to one embodiment of the presentinvention;

FIGS. 3A to 3D are cross-sectional views illustrating a process formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIG. 4 is a top view illustrating a process for manufacturing asemiconductor device according to one embodiment of the presentinvention;

FIGS. 5A to 5E are cross-sectional views illustrating a process formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 6A to 6D are cross-sectional views illustrating a process formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIG. 7 is a diagram illustrating one mode of an electronic device;

FIGS. 8A and 8B are each a diagram illustrating one mode of anelectronic device;

FIGS. 9A to 9C are graphs of results of TDS analysis;

FIGS. 10A to 10C are cross-sectional views each illustrating therelation between heat treatment and the amount of desorbed oxygen; and

FIGS. 11A to 11C are graphs showing electric characteristics of a thinfilm transistor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the drawings. Note that the present invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Therefore, the present invention should not be construed as beinglimited to the description in the following embodiments. Note that instructures of the present invention described hereinafter, like portionsor portions having similar functions are denoted by the same referencenumerals in different drawings, and description thereof is not repeated.

Note that in each drawing described in this specification, the size, thelayer (film) thickness, or the region of each component is exaggeratedfor clarity in some cases. Therefore, embodiments of the presentinvention are not limited to such scales illustrated in the drawings.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Embodiment 1

FIGS. 1A to 1E are cross-sectional views illustrating a manufacturingprocess of a transistor as one mode of a structure of a semiconductordevice. The cross-sectional view taken along the dot-dash line A-B inFIG. 2 corresponds to FIG. 1E.

As illustrated in FIG. 1A, an oxide insulating film 53 is formed over asubstrate 51, and an oxide semiconductor film 55 is formed over theoxide insulating film 53.

As the substrate 51, a glass substrate (also referred to as a“non-alkali glass substrate”), a quartz substrate, a ceramic substrate,a plastic substrate, or the like can be used as appropriate. Further, aflexible glass substrate or a flexible plastic substrate can be used asthe substrate 51. As a plastic substrate, a substrate having lowrefractive index anisotropy is preferably used, and a polyether sulfone(PES) film, a polyimide film, a polyethylene naphthalate (PEN) film, apolyvinyl fluoride (PVF) film, a polyester film, a polycarbonate (PC)film, an acrylic resin film, a prepreg which includes a fibrous body ina partially-cured organic resin, or the like can be typically used.

The oxide insulating film 53 is formed with use of an oxide insulatingfilm from which part of oxygen is desorbed by heat treatment. As such anoxide insulating film from which part of oxygen is desorbed by heattreatment, an oxide insulating film containing oxygen with a higherproportion than a proportion of oxygen in the stoichiometric compositionis preferably used. The oxide insulating film from which part of oxygenis desorbed by heat treatment can diffuse oxygen into the oxidesemiconductor film by heat treatment, because oxygen is desorbed fromthe oxide insulating film by heat treatment. Typical examples of theoxide insulating film 53 include films of silicon oxide, siliconoxynitride, silicon nitride oxide, aluminum oxide, aluminum oxynitride,gallium oxide, hafnium oxide, yttrium oxide, and the like.

Part of oxygen of the oxide insulating film containing oxygen with ahigher proportion than a proportion of oxygen in the stoichiometriccomposition is desorbed by heat treatment. The amount of desorbed oxygenis 1.0×10¹⁸ atoms/cm³ or more, preferably 1.0×10²⁰ atoms/cm³ or more,more preferably 3.0×10²⁰ atoms/cm³ or more at the oxygen atomicconversion by TDS analysis.

Here, in TDS analysis, the measurement method of the amount of desorbedoxygen at the oxygen atomic conversion is described below.

The desorption amount of gas in the TDS analysis is proportional to anintegral value of spectrum. Thus, from the ratio of the integral valueof spectrum of the oxide insulating film to a reference value of astandard sample, the desorption amount of gas can be calculated. Thereference value of the standard sample is a ratio of molecular densityto an integral value of spectrum of a sample containing given molecules.

For example, from a TDS analysis result of a silicon wafer containinghydrogen at a given density, which is a standard sample, and a TDSanalysis result of an oxide insulating film, the amount of oxygenmolecules (N_(O2)) desorbed from the oxide insulating film can beobtained by Equation 1.

N_(O2)=N_(H2)/S_(H2)×S_(O2)×α  (Equaltion 1)

N_(H2) is a value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample with density. S_(H2) is anintegral value of spectrum of a standard sample which is analyzed byTDS. In other words, the reference value of the standard sample isN_(H2)/S_(H2). S_(O2) is an integral value of spectrum when the oxideinsulating film is analyzed by TDS. α is a coefficient which influencesspectrum intensity in TDS analysis. As for the details of Equation 1,refer to JPH6-275697A. Note that the amount of oxygen desorbed from theoxide insulating film is measured with use of a silicon wafer containinghydrogen atoms at 1×10¹⁶ atoms/cm³ as a standard sample, by using athermal desorption spectrometer, EMD-WA1000S/W manufactured by ESCO Ltd.

In addition, N_(O2) is the amount of desorbed oxygen molecules (O₂). Inthe oxide insulating film, the amount of desorbed oxygen expressed inoxygen atom doubles the amount of desorbed oxygen expressed in oxygenmolecule (O₂).

The oxide insulating film 53 is 50 nm thick or more, preferably from 200nm to 500 nm. With use of the thick oxide insulating film 53, the amountof oxygen desorbed from the oxide insulating film 53 can be increased,and defects at the interface between the oxide insulating film 53 and anoxide semiconductor film to be formed later can be reduced.

The oxide insulating film 53 is formed by a sputtering method, a CVDmethod or the like. Preferably, the oxide insulating film from whichpart of oxygen is desorbed by heat treatment is easily formed by asputtering method.

When the oxide insulating film from which part of oxygen is desorbed byheat treatment is formed by a sputtering method, the amount of oxygencontained in a deposition gas is preferably large, and oxygen (O₂), amixed gas of oxygen (O₂) and a rare gas (such as Ar), or the like can beused. Typically, the oxygen concentration of a deposition gas ispreferably from 6% to 100%.

A silicon oxide film can be formed as a typical example of such an oxideinsulating film from which part of oxygen is desorbed by heat treatment.In that case, the silicon oxide film is preferably formed by a RFsputtering method under the following conditions: quartz (preferablysynthetic quartz) is used as a target; the substrate temperature is from30° C. to 450° C. (preferably from 70° C. to 200° C.); the distancebetween the substrate and the target (the T-S distance) is from 20 mm to400 mm (preferably from 40 mm to 200 mm); the pressure is from 0.1 Pa to4 Pa (preferably from 0.2 Pa to 1.2 Pa), the high-frequency power isfrom 0.5 kW to 12 kW (preferably from 1 kW to 5 kW); and the proportionof oxygen (O₂/(O₂+Ar)) in the deposition gas is from 1% to 100%(preferably from 6% to 100%). Note that a silicon target may be used asthe target instead of the quartz (preferably synthetic quartz) target.In addition, oxygen alone may be used as the deposition gas.

As the oxide semiconductor film 55, a four-component metal oxide such asan In—Sn—Ga—Zn—O-based metal oxide, a three-component metal oxide suchas an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide, atwo-component metal oxide such as an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, or an In—Mg—O-based metaloxide can be used. Here, an n-component metal oxide is constituted by nkinds of metal oxides. Preferably, the energy gap of the metal oxidewhich can form the oxide semiconductor film 55 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. In thismanner, off-state current of a transistor can be reduced by using ametal oxide having a wide band gap.

In addition, as the oxide semiconductor film, a thin film of a materialrepresented by the chemical expression, InMO₃(ZnO)_(m) (m>0), can beused. Here, M represents one or more metal elements selected from thegroup of Zn, Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Gaand Mn, Ga and Co, or the like.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target used has a composition ratio of In:Zn=50:1 to1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

Note that the oxide semiconductor film 55 contains hydrogen. Note thathydrogen may be contained in the oxide semiconductor film 55 in the formof a hydrogen molecule, water, a hydroxyl group, or hydride in somecases, in addition to a hydrogen atom.

The concentration of an alkali metal or an alkaline earth metalcontained in the oxide semiconductor film 55 is preferably 2×10¹⁶atoms/cm³ or less, or 1×10¹⁸ atoms/cm³ or less.

The thickness of the oxide semiconductor film 55 is preferably from 3 nmto 50 nm.

The oxide semiconductor film 55 can be formed by a sputtering method, acoating method, a printing method, a pulsed laser deposition method, orthe like can be used.

In this embodiment, the oxide semiconductor film 55 is formed by asputtering method using an In—Ga—Zn—O-based oxide target. Alternatively,the oxide semiconductor film 55 can be formed by a sputtering method ina rare gas (typically, argon) atmosphere, an oxygen atmosphere, or amixed atmosphere of a rare gas and oxygen.

As a sputtering gas used for forming the oxide semiconductor film 55, itis preferable to use a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed. When the oxidesemiconductor film 55 is formed when the substrate temperature is from100° C. to 600° C., preferably from 200° C. to 400° C., the impurityconcentration in the oxide semiconductor film 55 can be reduced.

Then heat treatment is conducted on the substrate 51 to remove hydrogenfrom the oxide semiconductor film 55, and to diffuse part of oxygencontained in the oxide insulating film 53 into the oxide semiconductorfilm 55 and into a region of the oxide insulating film 53 near theinterface between the oxide insulating film 53 and the oxidesemiconductor film 55

In this manner, deposition of the oxide semiconductor film is conductedwhile the heat treatment is conducted, or heat treatment is performedafter the deposition of the oxide semiconductor film, and thereby theoxide semiconductor film can have a region where crystals are alignedalong the C-axis.

A heat treatment apparatus used for the heat treatment is not limited toa particular apparatus, and the apparatus may be provided with a devicefor heating an object to be processed by heat radiation or heatconduction from a heating element such as a resistance heating element.For example, an electric furnace, or a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The temperature of the heat treatment is preferably a temperature atwhich hydrogen is removed from the oxide semiconductor film 55 and partof oxygen contained in the oxide insulating film 53 is desorbed and isdiffused into the oxide semiconductor film 55. The temperature istypically 150° C. or higher and lower than the strain point of thesubstrate 51, preferably 250° C. to 450° C.

The heat treatment is preferably conducted in an inert gas atmosphere;typically it is preferably performed in a rare gas (such as helium,neon, argon, xenon, or krypton) atmosphere or a nitrogen atmosphere.Alternatively, the heat treatment may be conducted in an oxidativeatmosphere.

This heat treatment can remove hydrogen from the oxide semiconductorfilm 55 and can diffuse a part of oxygen contained in the oxideinsulating film 53 into the oxide semiconductor film 55 and into aregion of the oxide insulating film 53 near the interface between theoxide insulating film 53 and the oxide semiconductor film 55. In thisprocess, oxygen vacancies in the oxide semiconductor film 55 can bereduced and oxygen is diffused into a region of the oxide insulatingfilm 53 near the interface of the oxide insulating film 53 and the oxidesemiconductor film 55, thereby reducing defects at the interface betweenthe oxide semiconductor film and the oxide insulating film. As a result,as illustrated in FIG. 1B, an oxide semiconductor film 57 with thelowered hydrogen concentration and the reduced oxygen vacancies can beformed.

The heat treatment is conducted when the oxide insulating film 53 iscovered with the oxide semiconductor film 55, so that part of oxygen inthe oxide insulating film 53 is diffused into the oxide semiconductorfilm 55. Thus, the oxygen vacancies of the oxide semiconductor film 55can be reduced. In addition, since the oxide insulating film 53 iscovered with the oxide semiconductor film 55 and the surface of theoxide insulating film 53 is not exposed, the amount of oxygen going outof the oxide insulating film 53 is reduced, so that defects at theinterface between the oxide insulating film 53 and the oxidesemiconductor film 55 can be efficiently reduced.

Next, after a mask is formed over the oxide semiconductor film 57, theoxide semiconductor film 57 is etched with the use of the mask to forman island-shaped oxide semiconductor film 59. After that, the mask isremoved (see FIG. 1C).

The mask used in the etching of the oxide semiconductor film 57 can beformed as appropriate by a photolithography process, an inkjet method, aprinting method, or the like. Wet etching or dry etching may be employedas appropriate for the etching of the oxide semiconductor film 57.

Next, as illustrated in FIG. 1D, a pair of electrodes 61 in contact withthe oxide semiconductor film 59 is formed.

At this time, the pair of electrodes 61 functions as a source electrodeand a drain electrode.

The pair of electrodes 61 can be formed using a metal element selectedfrom the group of aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing any of these metalelements as a component; an alloy containing these metal elements incombination; and the like. Further, one or more metal elements selectedfrom the group of manganese, magnesium, zirconium, and beryllium may beused. In addition, the pair of electrodes 61 can have a single-layerstructure or a stacked structure having two or more layers. For example,a single-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film, a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order, and the like can be given.

The pair of electrodes 61 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to employ a stacked-layer structure formedusing any of the above light-transmitting conductive materials and anyof the above metal elements.

The pair of electrodes 61 is formed by a printing method or an inkjetmethod. Alternatively, after a conductive film is formed by a sputteringmethod, a CVD method, an evaporation method or the like, a mask isformed over the conductive film and the conductive film is etched, andthereby the pair of electrodes 61 is formed. The mask formed over theconductive film can be formed by an inkjet method, a printing method, aphotolithography method, or the like as appropriate.

At this time, the conductive film is formed over the oxide semiconductorfilm 59 and the oxide insulating film 53, and etched into apredetermined pattern to form the pair of electrodes 61.

Note that the conductive film is formed over the oxide semiconductorfilm 57, a concavo-convex shaped mask is formed by a multi-tonephotomask, the oxide semiconductor film 57 and the conductive film areetched with use of the mask, and then the concavo-convex shaped mask isseparated by ashing, the conductive film is etched with use of theseparated masks to form the island-shaped oxide semiconductor film 59and the pair of electrodes 61. In this process, the number of thephotomasks used and the number of steps in the photolithography processcan be reduced.

Then, a gate insulating film 63 is formed over the oxide semiconductorfilm 59 and the pair of electrodes 61.

Next, a gate electrode 65 is formed in a region which is above the gateinsulating film 63 and overlaps with the oxide semiconductor film 59.

After that, an insulating film 69 may be formed as a protective film(FIG. 1E). In addition, after contact holes are formed in the gateinsulating film 63 and the insulating film 69, wirings connected to thepair of electrodes 61 may be formed.

The gate insulating film 63 can be formed with a single layer or astacked layer of silicon oxide, silicon oxynitride, silicon nitride,silicon nitride oxide, aluminum oxide, aluminum oxynitride, or galliumoxide. A portion of the gate insulating film 63 which is in contact withthe oxide semiconductor film 59 preferably contains oxygen,particularly, the gate insulating film 63 is preferably formed of asilicon oxide film. By using the silicon oxide film, it is possible todiffuse oxygen to the oxide semiconductor film 59, so that itscharacteristics can be improved.

When a high-k material such as hafnium silicate (HfSiO_(x)), hafniumsilicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafniumaluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z)), hafniumoxide, or yttrium oxide is used as the gate insulating film 63, thephysical thickness of the gate insulating film can be increased so thatgate leakage current can be reduced. Further, a stacked structure can beused in which a high-k material and one or more of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, and gallium oxide are stacked. For example,the thickness of the gate insulating film 63 is preferably from 1 nm to300 nm, and more preferably from 5 nm to 50 nm. In contrast, when thethickness of the gate insulating film 63 is 5 nm or more, gate leakagecurrent can be reduced.

Before the gate insulating film 63 is formed, the surface of theisland-shaped oxide semiconductor film 59 may be exposed to plasma of anoxidative gas such as oxygen, ozone, or dinitrogen monoxide so as to beoxidized, thereby reducing the oxygen vacancies.

The gate electrode 65 can be formed using a metal element selected fromthe group of aluminum, chromium, copper, tantalum, titanium, molybdenum,and tungsten; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; andthe like. Further, one or more metal elements selected from the group ofmanganese, magnesium, zirconium, and beryllium may be used. Further, thegate electrode 65 may have a single-layer structure or a stacked-layerstructure of two or more layers. For example, a single-layer structureof an aluminum film containing silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a two-layer structure inwhich a titanium film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a titaniumnitride film, a two-layer structure in which a tungsten film is stackedover a tantalum nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given.

The gate electrode 65 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to employ a stacked-layer structure formedusing any of the above light-transmitting conductive materials and anyof the above metal elements.

The insulating film 69 can be formed as appropriate with any of theinsulating films listed for the gate insulating film 63.

After that, heat treatment may be performed.

Through the above steps, a transistor having the oxide semiconductorfilm in a channel formation region can be formed.

In this embodiment, the heat treatment is performed when the surface ofthe oxide insulating film is covered with the oxide semiconductor film,and then the oxide semiconductor film is etched into a predeterminedshape to expose part of the oxide insulating film. Accordingly, theamount of oxygen going out of the oxide insulating film can besuppressed, part of oxygen can be diffused into the oxide semiconductorfilm to reduce oxygen vacancies in the oxide semiconductor film, andhydrogen can be desorbed from the oxide semiconductor film. As a result,as well as reduction of hydrogen and oxygen vacancies, which are carriersources of the oxide semiconductor film, reduction of defects at theinterface between the oxide semiconductor film and the oxide insulatingfilm can be achieved. Accordingly, the threshold voltage of thetransistor can be prevented from shifting to a negative side.

Embodiment 2

In Embodiment 2, a manufacturing method of a transistor having astructure different from the transistor described in Embodiment 1 willbe described with reference to FIGS. 3A to 3D, and FIG. 4. Embodiment 2is different from Embodiment 1 in that a pair of electrodes is providedbetween an oxide insulating film and an oxide semiconductor film. Notethat the cross-sectional view taken along the dot-dash line A-B in FIG.4 corresponds to FIG. 3D.

As illustrated in FIG. 3A, the oxide insulating film 53 is formed overthe substrate 51 as in Embodiment 1. A pair of electrodes 71 is formedover the oxide insulating film 53. An oxide semiconductor film 73 isformed over the pair of electrodes 71 and the oxide insulating film 53.

The pair of electrodes 71 can be formed as appropriate by using amaterial and by a formation method which are similar to those of thepair of electrodes 61 described in Embodiment 1.

The oxide semiconductor film 73 can be formed as appropriate by using amaterial and by a formation method which are similar to those of theoxide semiconductor film 55 described in Embodiment 1.

Then, as in Embodiment 1, the substrate 51 is heated so that an oxidesemiconductor film with the lowered hydrogen concentration and thereduced oxygen vacancies is formed. After that, a mask is formed overthe oxide semiconductor film with the lowered hydrogen concentration andthe reduced oxygen vacancies and etching is conducted on the oxidesemiconductor film with the lowered hydrogen concentration and thereduced oxygen vacancies to form an island-shaped oxide semiconductorfilm 75. Then, the mask is removed (see FIG. 3B).

As illustrated in FIG. 3C, a gate insulating film 77 is formed over thepair of electrodes 71 and the oxide semiconductor film 75. Then, a gateelectrode 79 is formed in a region which is above the gate insulatingfilm 77 and overlaps with the oxide semiconductor film 75. Then, aninsulating film 81 may be formed over the gate insulating film 77 andthe gate electrode 79 as a protective film.

The gate insulating film 77 can be formed as appropriate by using amaterial and by a formation method which are similar to those of thegate insulating film 63 described in Embodiment 1.

The gate electrode 79 can be formed as appropriate by using a materialand by a formation method which are similar to those of the gateelectrode 65 described in Embodiment 1.

The insulating film 81 can be formed as appropriate by using a materialand by a formation method which are similar to those of the insulatingfilm 69 described in Embodiment 1.

Then, after a mask is formed over the insulating film 81, the gateinsulating film 77 and the insulating film 81 are partially etched toform contact holes. Wirings 83 are formed so as to be connected to thepair of electrodes 71 through the contact holes.

The wirings 83 can be formed as appropriate by using a material and aformation method which are similar to those of the pair of electrodes71.

Through the above steps, the transistor having the oxide semiconductorfilm in a channel formation region can be formed.

In this embodiment, as well as reduction of hydrogen and oxygenvacancies, which are carrier sources of the oxide semiconductor film,reduction of defects at the interface between the oxide semiconductorfilm and the oxide insulating film can be achieved. Accordingly, thethreshold voltage of the transistor can be prevented from shifting to anegative side.

Embodiment 3

In Embodiment 3, a manufacturing method of a transistor where a contactresistance between an oxide semiconductor film and a pair of wirings canbe decreased more than those of the transistors described in Embodiment1 and Embodiment 2 will be described with reference to FIGS. 1A to 1Eand FIGS. 5A to 5E.

As in Embodiment 1, in the steps illustrated in FIGS. 1A and 1B, theoxide semiconductor film 55 is formed over the oxide insulating film 53and is heated to form the oxide semiconductor film 57 with the loweredhydrogen concentration and the reduced oxygen vacancies. Next, asillustrated in FIG. 5A, a buffer 84 having an n-type conductivity isformed over the oxide semiconductor film 57 with the lowered hydrogenconcentration and the reduced oxygen vacancies.

For the buffer 84 having an n-type conductivity, a metal oxide selectedfrom the group of indium oxide, indium tin oxide, indium zinc oxide, tinoxide, zinc oxide, and tin zinc oxide, or a material of the metal oxidewhich contains one or more elements selected from the group of aluminum,gallium, and silicon can be used. With such a structure, the contactresistance between the oxide semiconductor film and the pair ofelectrodes serving as a source electrode and a drain electrode to beformed later can be reduced.

In this case, when the oxide semiconductor film is heated to removehydrogen from the oxide semiconductor film, oxygen is concurrentlydiffused into the oxide semiconductor film from the oxide insulatingfilm. After that, the buffer 84 having an n-type conductivity is formedover the oxide semiconductor film, thereby sufficiently desorbinghydrogen from the oxide semiconductor film. As a result, the hydrogenconcentration and the oxygen vacancies in the oxide semiconductor filmcan be reduced, and thereby the threshold voltage of the transistor canbe prevented from shifting to a negative side.

Next, after a mask is formed over the buffer 84 having an n-typeconductivity, the oxide semiconductor film 57 with the lowered hydrogenconcentration and the reduced oxygen vacancies and the buffer 84 havingan n-type conductivity are etched to form the island-shaped oxidesemiconductor film 59 and an island-shaped buffer 85 having an n-typeconductivity. After that, the mask is removed (see FIG. 5B).

As illustrated in FIG. 5C, the pair of electrodes 61 is formed over theisland-shaped oxide semiconductor film 59 and the buffer 85 having ann-type conductivity. In this case, in order to keep the film quality ofthe gate insulating film, a material which does not extract oxygen fromthe gate insulating film is preferably used as the pair of electrodes61. Examples of the material of the pair of electrodes 61 includetungsten, molybdenum, and the like. However, tungsten or molybdenumunfortunately turns into a highly-resistant metal oxide in a regioncontacting with the oxide semiconductor film and the gate insulatingfilm. For that reason, the buffer having an n-type conductivity isprovided between the island-shaped oxide semiconductor film 59 and thepair of electrodes 61 so that the contact resistance between theisland-shaped oxide semiconductor film 59 and the pair of electrodes 61can be reduced.

Next, with use of a mask (not illustrated) formed over the pair ofelectrodes 61, an exposed portion of the buffer 85 having an n-typeconductivity is etched to form a pair of buffers 87 having an n-typeconductivity.

Note that it is possible that after the mask formed over the pair ofelectrodes 61 is removed, the pair of electrodes 61 is used as a maskand an exposed portion of the buffer 85 having an n-type conductivity isetched, so that the pair of buffers 87 having an n-type conductivity isformed.

When the buffer 85 having an n-type conductivity is etched, a conditionwhere the island-shaped oxide semiconductor film 59 is not etched andthe buffer 85 having an n-type conductivity is selectively etched (acondition with a high etching selectivity) is preferably adopted. Inaddition, if the etching selectivity between the island-shaped oxidesemiconductor film 59 and the buffer 85 having an n-type conductivity islow, the island-shaped oxide semiconductor film 59 is partially etchedinto a shape having a groove (a depressed portion) as well as the buffer85 having an n-type conductivity.

In this embodiment, because the pair of the buffers 87 having an n-typeconductivity are provided between the island-shaped oxide semiconductorfilm 59 and the pair of electrodes 61, the contact resistance betweenthe island-shaped oxide semiconductor film 59 and the pair of electrodes61 can be lowered. As a result, an on-state current of the transistorcan be prevented from being reduced.

Next, as in Embodiment 1, the gate insulating film 63, the gateelectrode 65 and the insulating film 69 are formed. In addition, aftercontact holes are formed in the gate insulating film 63 and theinsulating film 69, wirings may be formed so as to be connected to thepair of electrodes 61.

Through the above steps, the transistor having the oxide semiconductorfilm in a channel formation region can be formed.

In this embodiment, because the oxide semiconductor film with thelowered hydrogen concentration and the reduced oxygen vacancies isformed and the buffer having an n-type conductivity is formed to reducethe contact resistance between the oxide semiconductor film and the pairof wirings, the threshold voltage of the transistor can be preventedfrom shifting to a negative side and the on-state current of thetransistor can be prevented from being reduced.

Embodiment 4

In Embodiment 4, a transistor having a large amount of on-state currentand high field effect mobility or a transistor with a controlledthreshold voltage will be described with reference to FIGS. 6A to 6D.

As illustrated in FIG. 6A, the oxide insulating film 53 is formed overthe substrate 51. A first gate electrode 91 is formed over the oxideinsulating film 53. A first gate insulating film 93 is formed over theoxide insulating film 53 and the first gate electrode 91. An oxidesemiconductor film 95 is formed over the first gate insulating film 93.

The first gate electrode 91 can be formed in a manner similar to that ofthe gate electrode 65 in Embodiment 1.

The first gate insulating film 93 can be formed in a manner similar tothat of the gate insulating film 63 in Embodiment 1.

The oxide semiconductor film 95 can be formed in a manner similar tothat of the oxide semiconductor film 55 in Embodiment 1.

Next, as illustrated in FIG. 6B, as in Embodiment 1, the oxidesemiconductor film 95 is heated to form an oxide semiconductor film 97with the lowered hydrogen concentration and the reduced oxygenvacancies.

Then, a mask is formed over the oxide semiconductor film 97 and theoxide semiconductor film 97 is etched to form an island-shaped oxidesemiconductor film 99. After that, the mask is removed (see FIG. 6C).

Next, as illustrated in FIG. 6D, a pair of electrodes 101 is formed overthe island-shaped oxide semiconductor film 99. Then, a second gateinsulating film 103 is formed over the island-shaped oxide semiconductorfilm 99 and the pair of electrodes 101. A second gate electrode 105 isformed in a region which is above the second gate insulating film 103and overlaps with the island-shaped oxide semiconductor film 99. Aninsulating film 109 may be formed over the second gate insulating film103 and the second gate electrode 105 as a protective film.

The pair of electrodes 101 can be formed in a manner similar to that ofthe pair of electrodes 61 described in Embodiment 1.

The second gate insulating film 103 can be formed in a manner similar tothat of the gate insulating film 63 in Embodiment 1.

The second gate electrode 105 can be formed in a manner similar to thatof the gate electrode 65 described in Embodiment 1.

The insulating film 109 can be formed in a manner similar to that of theinsulating film 69 described in Embodiment 1.

The first gate electrode 91 and the second gate electrode 105 may beconnected. In this case, the first gate electrode 91 and the second gateelectrode 105 have the same potential and a channel formation region isformed on the first gate insulating film 93 side and on the second gateinsulating film 103 side of the oxide semiconductor film 99, and therebythe on-state current and field effect mobility of the transistor can beincreased.

Alternatively, it is also possible that the first gate electrode 91 andthe second gate electrode 105 are not connected and have differentapplied potentials. In this case, the threshold voltage of thetransistor can be controlled.

In this embodiment, the pair of electrodes is formed between theisland-shaped oxide semiconductor film 99 and the second gate insulatingfilm 103, but the pair of electrodes may be formed between the firstgate insulating film 93 and the island-shaped oxide semiconductor film99.

Through the above-described steps, the transistor having a plurality ofgate electrodes can be formed.

Embodiment 5

In Embodiment 5, a method which can lower the hydrogen concentration ofan oxide semiconductor film more than the methods described inEmbodiments 1 to 4 will be described. Note that description ofEmbodiment 5 is made with reference to Embodiment 1; however, Embodiment5 can be applied to Embodiments 2 to 4 as appropriate.

In FIG. 1A, the substrate 51 is heated before the oxide insulating film53 is formed over the substrate 51. The oxide insulating film 53 and theoxide semiconductor film 55 are formed over the substrate 51.

The heating temperature of the substrate 51 is preferably a temperaturewhich enables desorption of hydrogen adsorbed to or contained in thesubstrate 51. Typically, the temperature is 100° C. or higher and lowerthan a strain point of the substrate. In addition, the heat treatment onthe substrate 51 is preferably conducted in an atmosphere with lowhydrogen content. Preferably, it is conducted in high vacuum of 1×10⁻⁴Pa or less. As a result, hydrogen, hydrogen molecules, water, a hydroxylgroup, hydride etc., adsorbed on the surface of the substrate can bereduced.

Further, a series of steps of the heat treatment on the substrate 51 tothe formation of the oxide semiconductor film 55 is performedcontinuously in vacuum without exposure to air. Therefore, hydrogen,hydrogen molecules, water, a hydroxyl group, hydride etc., are notadsorbed on the substrate 51, the oxide insulating film 53 and the oxidesemiconductor film 55, and in the heat treatment on the oxidesemiconductor film 55, hydrogen diffusion into the oxide semiconductorfilm 55 from the substrate 51 and the oxide insulating film 53 can besuppressed, and thus the hydrogen concentration of the oxidesemiconductor film 57 which is heated, illustrated in FIG. 1B can belowered. As a result, the threshold voltage of the transistor can beprevented from shifting to a negative side.

Embodiment 6

The transistor described in any of Embodiments 1 to 5 is formed, and asemiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor for a pixelportion and further for a driver circuit. Further, part of or the entiredriver circuit including transistors can be formed over a substratewhere the pixel portion is formed; thus, a system-on-panel can beobtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or a voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Furthermore, an element substrate,which corresponds to one mode before the display element is completed ina manufacturing process of the display device, is provided with meansfor supplying current to the display element in each pixel.Specifically, the element substrate may have a mode in which only apixel electrode of the display element is provided, a mode after aconductive film to be a pixel electrode is formed and before the pixelelectrode is formed by etching the conductive film, or any other states.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes the following modules inits category: a module including a connector such as a flexible printedcircuit (FPC), a tape automated bonding (TAB) tape, or a tape carrierpackage (TCP) attached; a module having a TAB tape or a TCP at the endof which a printed wiring board is provided; and a module having anintegrated circuit (IC) which is directly mounted on a display elementby a chip on glass (COG) method.

Embodiment 7

A semiconductor device disclosed in this specification can be applied toelectronic paper. Electronic paper can be used for electronic devices,which can display data, of a variety of fields. For example, electronicpaper can be applied to an electronic book reader (e-book), a poster, adigital signage, a public information display (PID), an advertisement ina vehicle such as a train, displays of various cards such as a creditcard, and the like. An example of the electronic device is illustratedin FIG. 7.

FIG. 7 illustrates an electronic book reader 2700 as one example of suchelectric appliances. For example, the e-book reader 2700 includes twohousings, a housing 2701 and a housing 2703. The housing 2701 and thehousing 2703 are combined with a hinge 2711 so that the e-book reader2700 can be opened and closed with the hinge 2711 as an axis. With sucha structure, the e-book reader 2700 can operate like a paper book.

A display portion 2705 and a photoelectric conversion device 2706 areincorporated in the housing 2701. A display portion 2707 and aphotoelectric conversion device 2708 are incorporated in the housing2703. The display portion 2705 and the display portion 2707 may displayone image or different images. In the case where the display portion2705 and the display portion 2707 display different images, for example,text data can be displayed on a display portion on the right side (thedisplay portion 2705 in FIG. 7) and image data can be displayed on adisplay portion on the left side (the display portion 2707 in FIG. 7).

FIG. 7 illustrates an example in which the housing 2701 is provided withan operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turnedover. Note that a keyboard, a pointing device, or the like may also beprovided on the surface of the housing, on which the display portion isprovided. Furthermore, an external connection terminal (an earphoneterminal, a USB terminal, a terminal that can be connected to an ACadapter or various cables such as a USB cable, or the like), a recordingmedium insertion portion, and the like may be provided on the backsurface or the side surface of the housing. Moreover, the e-book reader2700 may have a function of an electronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Embodiment 8

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 8A illustrates a television set 9600 as one example of suchelectronic devices. In the television set 9600, a display portion 9603is incorporated in a housing 9601. The display portion 9603 can displayimages. Here, the housing 9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 8B illustrates a digital photo frame 9700 as one example of suchelectronic devices. For example, in the digital photo frame 9700, adisplay portion 9703 is incorporated in a housing 9701. The displayportion 9703 can display a variety of images. For example, the displayportion 9703 can display data of an image taken with a digital camera orthe like and function as a normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalthat can be connected to various cables such as a USB cable, or thelike), a recording medium insertion portion, and the like. Althoughthese components may be provided on the surface on which the displayportion is provided, it is preferable to provide them on the sidesurface or the back surface for the design of the digital photo frame9700. For example, a memory storing data of an image taken with adigital camera is inserted in the recording medium insertion portion ofthe digital photo frame, whereby the image data can be transferred andthen displayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly to be displayed.

Example 1

In Example 1, the formation process of an oxide insulating film and anoxide semiconductor film, and the relation between the timing of heattreatment and the amount of oxygen desorbed from the oxide insulatingfilm will be described with reference to FIGS. 9A to 9C, FIGS. 10A to10C, and FIGS. 11A to 11C.

First, a method of forming each sample will be described. In Example 1,at least one step of Steps 1 to 4 below is conducted to form samples.

(Step 1) An oxide insulating film is formed over a substrate. At thistime, a 300-nm-thick silicon oxide film is formed as the oxideinsulating film.

The oxide insulating film is formed by an RF sputtering method underconditions that a quartz target is used, argon with a flow rate of 25sccm and oxygen with a flow rate of 25 sccm are used as sputteringgases, the power of a high frequency power source with a frequency of13.56 MHz is 1.5 kW, the pressure is 0.4 Pa, the distance between thesubstrate and the target is 60 mm, and the substrate temperature is 100°C.

(Step 2) An oxide semiconductor film is formed over the oxide insulatingfilm. At this time, a 30-nm-thick In—Ga—Zn—O film is formed as the oxidesemiconductor film.

The oxide semiconductor film is formed by a DC sputtering method underconditions that an In—Ga—Zn—O (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio])target is used, argon with a flow rate of 30 sccm and oxygen with a flowrate of 15 sccm are used as sputtering gases, the power of the powersource is 0.5 kW, the pressure is 0.4 Pa, the distance between thesubstrate and the target is 60 mm, and the substrate temperature is 200°C.

(Step 3) A mask is formed over the oxide semiconductor film by aphotolithography process, and then the oxide semiconductor film ispartially etched to form an island-shaped oxide semiconductor film.(Step 4) Heat treatment is conducted.

The heat treatment is conducted as follows. Samples are heated at 450°C. in an electric furnace with a nitrogen atmosphere and held for 1hour.

Sample 1, Sample 3, and Sample 5 are not subjected to the heat treatmentwhile Sample 2, Sample 4, and Sample 6 are subjected to the heattreatment. Further, Sample 2, Sample 4, and Sample 6 are subjected tothe heat treatment at different timings.

In Sample 1, the oxide semiconductor film is removed after Step 1 andStep 2. In Sample 2, the oxide semiconductor film is removed after Step4 (heat treatment) is conducted following Step 1 and Step 2.

As for Sample 3, Step 1 is conducted. As for Sample 4, Step 4 (heattreatment) is conducted after Step 1.

In Sample 5, the island-shaped oxide semiconductor film is removed afterSteps 1 to 3. In Sample 6, the island-line oxide semiconductor film isremoved after Step 4 (heat treatment) is conducted following Steps 1 to3.

Next, the degassing yields of Samples 1 to 6 are measured by TDSanalysis. A thermal desorption spectrometer EMD-WA1000S/W, manufacturedby ESCO Ltd., was used for the TDS analysis. Note that TDS analysis isan analysis method in which a sample is heated in a vacuum case and agas component generated from the sample when the temperature of thesample is increased is detected by a quadrupole mass analyzer. Detectedgas components are distinguished from each other by the value of m/z(mass/charge). TDS spectra when m/z is 32 are shown here. Note thatthere are an oxygen molecule (O₂) and the like as a component of m/z of32.

TDS spectra of Sample 1 (not heated) and Sample 2 (heated) are shown bya broken line 201 and a solid line 203 respectively in FIG. 9A.

TDS spectra of Sample 3 (not heated) and Sample 4 (heated) are shown bya broken line 205 and a solid line 207 respectively in FIG. 9B.

TDS spectra of Sample 5 (not heated) and Sample 6 (heated) are shown bya broken line 209 and a solid line 211 respectively in FIG. 9C.

FIGS. 10A, 10B, and 10C are views illustrating models of oxygen releasedfrom oxide insulating films 113 at the heat treatment for Samples 2, 4,and 6 respectively.

As illustrated in FIG. 10A, the oxide insulating film 113 and an oxidesemiconductor film 115 stacked over a substrate 111 are heated so thatthe oxide semiconductor film 115 serves as a protective film to suppressoxygen release out of the oxide insulating film 113. For that reason, asillustrated in FIG. 9A, there is almost no difference in the amount ofreleased oxygen with or without heat treatment in TDS analysis in Sample1 and Sample 2 each of which oxide semiconductor film is removed. Thisis because the amount of oxygen, which can be desorbed by heattreatment, contained in the oxide insulating film is not reduced somuch.

On the other hand, as illustrated in FIG. 10B, heat treatment isconducted when an oxide semiconductor film covering the surface of theoxide insulating film 113 formed over the substrate 111 is not formed,oxygen is released from the oxide insulating film 113. For that reason,as shown by the solid line 207 in FIG. 9B, the amount of released oxygenof Sample 4 which is heated is smaller in TDS analysis than that ofSample 3. This is because part of oxygen is released out of the oxideinsulating film by heat treatment, and the amount of oxygen, which canbe desorbed by heat treatment, contained in the oxide insulating film isreduced.

Further, as illustrated in FIG. 10C, heat treatment is conducted when anisland-shaped oxide semiconductor film 117 covering part of the surfaceof oxide insulating film 113 formed over the substrate 111 is formed, sothat oxygen is released from an exposed portion of the oxide insulatingfilm 113. In addition, oxygen contained in the oxide insulating film 113in contact with the island-shaped oxide semiconductor film 117 isdiffused and released from the exposed portion of the oxide insulatingfilm 113. This is because the diffusion coefficient of oxygen in theoxide insulating film 113 is larger than the diffusion coefficient ofoxygen from the oxide insulating film 113 into the oxide semiconductorfilm and thus the diffusion speed inside the oxygen insulating film isfaster than the diffusion speed of oxygen from the oxide insulating filminto the oxide semiconductor film, so that oxygen is diffused in theperiphery of the island-shaped oxide semiconductor film. Specifically,it is confirmed by analysis that the diffusion coefficient of oxygen inthe oxide insulating film 113 at the heat treatment of 450° C. is about1×10⁻¹⁰ cm²/sec and the diffusion coefficient of oxygen in the oxidesemiconductor film 117 at the heat treatment of 450° C. is about 1×10⁻¹⁷cm²/sec. As for the diffusion coefficient of oxygen in the oxidesemiconductor film 117, the value of an In—Ga—Zn—O film as the oxidesemiconductor film is shown and other oxide semiconductors show similartendency. For example, the diffusion coefficient of oxygen in an In—Sn—Ofilm is about 6×10⁻¹⁶ cm²/sec, and is much smaller than the diffusioncoefficient of oxygen in the oxide insulating film 113. Thus, as shownby the solid line 211 in FIG. 9C, the amount of released oxygen in TDSanalysis of Sample 6 which is heated is smaller than that of Sample 5.This is because part of oxygen is released out of the oxide insulatingfilm and the amount of oxygen, which can be desorbed by heat treatment,contained in the oxide insulating film is reduced.

As described above, heat treatment is conducted at the phase where theoxide semiconductor film 115 is formed over the oxide insulating film113 and the island-shaped oxide semiconductor film 117 is not yetformed, and thereby oxygen contained in the oxide insulating film 113can be diffused efficiently into the oxide semiconductor film 115.

Example 2

In Example 2, a formation method and electric characteristics of atransistor will be described with reference to FIGS. 1A to 1E and FIGS.11A to 11C.

In Example 2, similarly to Example 1, Samples 7 to 9 are formed in stepsof heat treatment conducted at different timings. Sample 7 is subjectedto heat treatment after an oxide semiconductor film is formed over anoxide insulating film. Sample 8 is subjected to heat treatment after anoxide insulating film is formed. Sample 9 is subjected to heat treatmentafter an oxide semiconductor film is formed over an oxide insulatingfilm, and part of the oxide semiconductor film is etched to form anisland-shaped oxide semiconductor film.

First, a method for forming each of Samples 7 to 9 is described.

As illustrated in FIG. 1A, the oxide insulating film 53 is formed overthe substrate 51. The oxide semiconductor film 55 is formed over theoxide insulating film 53. Note that Sample 8 is subjected to heattreatment after the oxide insulating film 53 is formed and before theoxide semiconductor film 55 is formed. In addition, Sample 7 issubjected to heat treatment after the oxide semiconductor film 55 isformed and before the oxide semiconductor film 59 illustrated in FIG. 1Cis formed.

A glass substrate AN100 (manufactured by Asahi Glass Co., Ltd.) is usedas the substrate 51.

The deposition conduction of the oxide insulating film 53 is thedeposition condition of the oxide insulating film described in Step 1 ofExample 1 and a silicon oxide film with a thickness of 300 nm is formed.

The deposition conduction of the oxide semiconductor film 55 is thedeposition condition of the oxide semiconductor film described in Step 2of Example 1 and an In—Ga—Zn—O film with a thickness of 30 nm is formed.

The heat treatment on Samples 7 and 8 is conducted with the condition ofthe heat treatment described in Step 4 of Example 1.

Then, after a mask is formed over the oxide semiconductor film by aphotolithography process, the oxide semiconductor film is etched withuse of the mask. Note that Sample 9 is subjected to heat treatment afterthe oxide semiconductor film is etched. Through the above steps, theoxide semiconductor film 59 is formed as illustrated in FIG. 1C.

At this time, wet etching is employed to etch the oxide semiconductorfilm.

The heat treatment on Sample 9 is conducted with the condition of theheat treatment described in Step 4 of Example 1.

Then, the pair of electrodes 61 is formed over the oxide semiconductorfilm 59 (FIG. 1D).

In this case, a 100-nm-thick tungsten film is formed as a conductivefilm by a sputtering method. Then, a mask is formed over the conductivefilm by a photolithography process and the conductive film is dry-etchedwith use of the mask to form the pair of electrodes 61. After that, themask is removed.

Then, the gate insulating film 63 is formed over the oxide semiconductorfilm 59 and the pair of electrodes 61. The gate electrode 65 is formedover the gate insulating film 63. The insulating film 69 serving as aprotective film is formed (FIG. 1E).

As the gate insulating film 63, a silicon oxynitride film is formed by aplasma CVD method. In Sample 7 and Sample 8, the gate insulating filmhas a thickness of 30 nm while in Sample 9, the gate insulating film hasa thickness of 15 nm.

Further, over the gate insulating film 63, a 15-nm-thick tantalumnitride film is formed by a sputtering method and then a 135-nm-thicktungsten film is formed by a sputtering method. Then, a mask is formedby a photolithography process, then the tungsten film and the tantalumnitride film are dry-etched with use of the mask to form the gateelectrode 65, and then the mask is removed.

In this case, as the insulating film 69, a silicon oxynitride film witha thickness of 300 nm is formed by a plasma CVD method.

Next, although not illustrated, a mask is formed over the insulatingfilm 69 by a photolithography process and part of the insulating film 69is etched with use of the mask to form a contact hole. At this time, thegate insulating film 63 and the insulating film 69 are dry-etched toform contact holes where the pair of electrodes 61 and the gateelectrode 65 are exposed.

Then, wirings connecting to the pair of electrodes 61 and the gateelectrode 65 are formed.

Here, a 50-nm-thick titanium film, a 100-nm-thick aluminum film and a5-nm-thick titanium film are sequentially formed over the insulatingfilm 69 by a sputtering method. Then, a mask is formed by aphotolithography process and the titanium film, the aluminum film, andthe titanium film are dry-etched with use of the mask to form a wiring,and then the mask is removed.

Then, Samples are heated in an electric furnace with a nitrogenatmosphere at 250° C. and held for 1 hour.

Through the above-described steps, the transistors are formed. Then,electric characteristics of the transistors formed in Samples 7 to 9 aremeasured. The measurement results of the 25 points are all shown. Thechannel length L is 3 μm, and the channel width W is 10 μl.

FIGS. 11A, 11B, and 11C show electric characteristics of the transistorsformed in Sample 7, Sample 8, and Sample 9 respectively. Curves 221,225, and 229 are current-voltage curves when a drain voltage of 0.1 V isapplied to each transistor, while curves 223, 227, and 231 arecurrent-voltage curves when a drain voltage of 3 V is applied to eachtransistor.

The current-voltage curves shown in FIG. 11A exhibits high on/off ratioand switching characteristics are obtained, whereas the current-voltagecurves shown in FIGS. 11B and 11C do not exhibit on/off ratio andswitching characteristics are not obtained.

From the TDS analysis results in Example 1, as shown with Sample 2, evenwhen the heat treatment is conducted in the state where the surface ofthe oxide insulating film is covered with the oxide semiconductor film,the amount of oxygen, which can be desorbed by heat treatment, containedin the oxide insulating film is not reduced so much, and thus release ofoxygen hardly occurs. On the other hand, with regards to Sample 4 andSample 6, the heat treatment is conducted in the state that the oxideinsulating film is exposed and thus oxygen is released outside.

Based on the above description, in Sample 7 of Example 2 which issubjected to the heat treatment at the same timing of heat treatment asSample 2, part of oxygen contained in the oxide insulating film isdiffused into the oxide semiconductor film by the heat treatment, andoxygen release to the outside is suppressed, so that the number ofoxygen vacancies in the oxide semiconductor film is reduced. Therefore,the transistor using the oxide semiconductor film has switchingcharacteristics.

On the other hand, with regards to Sample 8 and Sample 9 of Example 2which are subjected to heat treatment at the same timings of heattreatment as Sample 4 and Sample 6, part of oxygen contained in theoxide insulating film is released outside by the heat treatment, andthus the amount of diffused oxygen into the oxide semiconductor film issmall and many oxygen vacancies serving as carrier sources are present.Therefore, the oxide semiconductor film has an n-type conductivity andthus a transistor using the oxide semiconductor film does not haveswitching characteristics.

Based on the above description, the heat treatment is conducted in thestate that the surface of the oxide insulating film, from which part ofoxygen is desorbed by heat treatment, is covered with the oxidesemiconductor film, then etching of the oxide semiconductor film into apredetermined shape is conducted, and the gate insulating film and thegate electrode are formed, whereby a transistor with sufficiently highon/off ratio and little shift to a negative side of a threshold voltagecan be provided.

This application is based on Japanese Patent Application serial no.2010-181832 filed with Japan Patent Office on Aug. 16, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductor devicecomprising the steps of: forming an oxide insulating film over asubstrate; forming an oxide semiconductor film over the oxide insulatingfilm; performing a heat treatment after forming the oxide semiconductorfilm; etching a part of the oxide semiconductor film to form anisland-shaped oxide semiconductor film after performing the heattreatment; forming a pair of electrodes over the island-shaped oxidesemiconductor film; forming a gate insulating film over theisland-shaped oxide semiconductor film and the pair of electrodes; andforming a gate electrode overlapping with the island-shaped oxidesemiconductor film with the gate insulating film therebetween.